Evidence for Anisotropic Vortex Dynamics and Pauli Limitation in the Upper Critical Field of FeSe1-xTex

We have determined H-C2(T) for FeSe1-xTex (x = 0.52) single crystals using resistivity measurements at high static and pulsed magnetic field, as well as specific heat measurements up to 9 T. We find that the significant anisotropy of the initial slope of H-C2(T) determined from resistivity measurements, is not present when H-C2 is determined from the specific heat results. This suggests that the thermodynamic upper critical field is almost isotropic, and that anisotropic vortex dynamics play a role. Further evidence of anisotropic vortex dynamics is found in the behaviour in pulsed field. We also find that Pauli limiting must be included in order to fit the temperature dependence of H-C2, indicating probably higher effective mass in FeSe1-xTex than in other Fe superconductors.